REPORT ON THE DEEP-SEA KERATOSA. 43 



spicules of various sponges, &c.). These are disposed in the horny fibres of the skeleton, 

 but sometimes also in the clear maltha or the ground-mass of the mesoderm. Sometimes 

 the spongin is developed very scantily, and forms only thin sheaths, partially covering 

 the xenophya connected by it, or saccular envelopes around them. 



The external form in the Spongelidse is very variable, as also in the Euspongidse. 

 The canal-system is formed on the Leuconal-type (the third type of Vosmaer), with 

 roundish or oblongish flagellated chambers of variable size, usually rather large, but 

 sometimes very small. It is impossible to retain the relative size of the flagello-chambers 

 as the essential difference between the Spongelidse and Euspongidse. Among the Deep- 

 sea Keratosa collected by the Challenger, there are five distinct species belonging to the 

 Spongelidse. They represent two different new genera, both of special interest. Their 

 peculiar organisation is probably due (to a certain extent at least) to the symbiosis with 

 a Hydroid, the reticular hydrorhiza of which traverses the whole body of these sponges. 



The first genus, Cerelasma (PI. VI.), is distinguished from all other Spongelidse (and 

 probably from all other Keratosa hitherto described) by the peculiar mode of the spongin- 

 secretion. The yellow horny substance of the skeleton forms in the two species of this 

 genus not a framework of anastomosing cylindrical fibres, as usual, but saccular 

 envelopes around the innumerable xenophya which compose the pseudo-skeleton ; 

 these are connected by irregular branched lamellse, which are expanded in the meshes 

 between the branches of the symbiotic hydrorhiza. The sponge itself represents in the 

 two species of Cerelasma a globular or tuberose body composed of numerous anastomosing 

 branches, which are either lamellar or cylindrical. 



The second genus, Psammophyllum (Pis. IV., V.), is represented by three species, 

 which are very similar in external shape to the Stannomid genus Stannophyllum 

 (Pis. L, II.). The body is invariably a pedunculated flabelliform leaf. Its spongy 

 substance is supported by the reticular hydrorhiza of a symbiotic Hydroid, and 

 overladen with xenophya. But the essential difference between the two similar genera 

 is, that the simple (rarely branched) spongin-fibrillse of Stannophyllum do not 

 anastomose, form no network, and do not include the xenophya. In Psammophyllum, 

 however, as in all true Spongelidse, the anastomosing spongin-fibres form a network, and 

 include (partially or totally) the foreign bodies of the pseudo-skeleton. 



Psammophyllum is closely allied to that remarkable Spongelid described by Esper 

 as Spongia papyracea, 1 by Ehlers 2 and Hyatt 3 as Phyllospongia papyracea. But if the 

 description of this latter be correct, it differs from Psammophyllum in two essential 

 points. The two sides of the flabelliform leaf are the same in Psammophyllum, whereas 

 in Phyllospougia the upper and lower sides have a very different structure. In the 



1 Esper, Spongien, Forts., Bd. ii. p. 38, Taf. lxv. 



2 Ehlers, Die Esperschen Spongien, pp. 22, 30, 1870. 



3 Hyatt, Revision North Amer. Porif., part ii. p. 73, pi. xvii. fig. 31, 1876. 



